<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <ul id="box">
    <li>1</li>
    <li>2</li>
    <li>3</li>
  </ul>

  <ol>
    <li>q</li>
    <li>w</li>
    <li>e</li>
  </ol>

  <script>
    // function fn() {
    //   console.log( arguments )  // 伪数组
    // }

    var box = document.getElementById('box')

    var list = box.getElementsByTagName('li')  //  伪数组, dom列表，  arguments

    for(var i = 0; i < list.length; i++) {
      console.log( list[i] )
    }

  </script>
</body>
</html>